To cope with the phenomenal growth in wireless data traffic, scarce radio resources are being aggressively reused in cellular communication networks. Mutual interference among users occupying the same radio channel has thus become a major performance impediment in cellular communications. Conventional wireless receiver designs focus on combating background thermal noise to improve receiver sensitivity and typically model co-channel interference in a similar fashion as the thermal noise based on a Gaussian probability distribution. However, since the number of significant interferers in a cellular communication network is typically quite small, the statistics of co-channel interference can be rather different from the Gaussian probability distribution. As a result, the performance of conventional receivers is often far from the optimum achievable in interference-limited situations. Improved wireless receiver designs that can better exploit the interference statistics are therefore desirable.
More specifically, in conventional wireless receivers, the interference and the thermal noise are typically modeled collectively as colored Gaussian noise. This leads to the use of a demodulation metric that depends only on second-order statistics of the interference. For instance, a conventional baseband model of a desired signal received at a wireless receiver equipped with one or more receive antennas is:r= HPs+v=Hs+v, where r=(r1, r2, . . . , rnR) denotes a signal vector for a particular channel use (e.g., for a particular subcarrier in a particular time slot in an Orthogonal Frequency Division Multiplexing (OFDM) transmission) of a desired signal through nR receive antennas, v denotes an impairment signal that includes both interference and thermal noise, H denotes a nR-by-nT Multiple-Input-Multiple-Output (MIMO) channel matrix, P denotes a nT-by-nS precoding matrix for mapping a symbol vector s=[s1, s2, . . . snS]Tε(ΛD)nS of nS streams of desired information symbols into nT transmit antennas, ΛD denotes a set of constellation points in a modulation constellation of a modulation used for the desired signal, and H denotes an nR-by-nS effective MIMO channel matrix. Traditionally, the impairment signal v is modeled as colored Gaussian noise with a probability distribution given by:
                    p        v            ⁡              (                  v          0                )              ≡                  1                              π                          n              R                                ⁢                      det            ⁡                          (                              R                v                            )                                          ⁢      exp      ⁢              {                              -                          v              0              H                                ⁢                      R            v                          -              1                                ⁢                      v            0                          }              ,where Rv≡E[vvH] denotes an covariance matrix of the impairment signal v, which is referred to herein as an impairment covariance matrix. This leads to the use of the conventional Euclidean-distance-based demodulation metric (mconv) given by:
                                                                                          m                  conv                                ⁡                                  (                                                            s                      |                      r                                        ,                                          R                      v                                        ,                    H                                    )                                            ≡                            ⁢                                                -                  ln                                ⁢                                                                  ⁢                                  p                  ⁡                                      (                                          r                      |                      s                                        )                                                                                                                          =                            ⁢                                                -                  ln                                ⁢                                                                  ⁢                                                      p                    v                                    ⁡                                      (                                          r                      -                      Hs                                        )                                                                                                                                          =                                ⁢                                                                                                    (                                                  r                          -                          Hs                                                )                                            H                                        ⁢                                                                  R                        v                                                  -                          1                                                                    ⁡                                              (                                                  r                          -                          Hs                                                )                                                                              +                                      ln                    ⁢                                                                                  ⁢                                          π                                              n                        R                                                              ⁢                                          det                      ⁡                                              (                                                  R                          v                                                )                                                                                                        ,                                                          (        1        )            where the last term is independent of s and can be omitted. This metric can then be used to demodulate the symbol vector by computing:
      s    *    =                    arg        ⁢                                  ⁢        min            s        ⁢                  m        conv            ⁡              (                              s            |            r                    ,          R          ,          H                )              ⁢                  arg        ⁢                                  ⁢        min            s        ⁢                  (                  r          -          Hs                )            H        ⁢                            R          v                      -            1                          ⁡                  (                      r            -            Hs                    )                    .      
The conventional metric (mconv) in Equation (1) leads to the conventional formula for computing soft bit information for, say, the i-th bit of symbol sk given by:
                                                                        β                                  k                  ,                  i                                conv                            ≡                            ⁢                              ln                ⁢                                                      p                    ⁡                                          (                                                                        b                                                      k                            ,                            i                                                                          =                                                  1                          |                          r                                                                    )                                                                            p                    ⁡                                          (                                                                        b                                                      k                            ,                            i                                                                          =                                                                              -                            1                                                    |                          r                                                                    )                                                                                                                                              =                            ⁢                                                ln                  ⁢                                                                                    ∑                                                                              s                            ⁢                                                          :                                                        ⁢                                                          b                                                              k                                ,                                i                                                                                                              =                          1                                                                    ⁢                                              p                        ⁡                                                  (                                                      r                            |                            s                                                    )                                                                                                                                    ∑                                                                              s                            ⁢                                                          :                                                        ⁢                                                          b                                                              k                                ,                                i                                                                                                              =                                                      -                            1                                                                                              ⁢                                              p                        ⁡                                                  (                                                      r                            |                            s                                                    )                                                                                                                    +                                  ln                  ⁢                                                            p                      ⁡                                              (                                                                              b                                                          k                              ,                              i                                                                                =                          1                                                )                                                                                    p                      ⁡                                              (                                                                              b                                                          k                              ,                              i                                                                                =                                                      -                            1                                                                          )                                                                                                                                                                    =                            ⁢                                                ln                  ⁢                                                            ∑                                                                        s                          ⁢                                                      :                                                    ⁢                                                      b                                                          k                              ,                              i                                                                                                      =                        1                                                              ⁢                                          exp                      ⁢                                              {                                                  -                                                                                    m                              conv                                                        ⁡                                                          (                                                                                                s                                  |                                  r                                                                ,                                                                  R                                  v                                                                ,                                H                                                            )                                                                                                      }                                                                                            -                                                                                                      ⁢                                                ln                  ⁢                                                            ∑                                                                        s                          ⁢                                                      :                                                    ⁢                                                      b                                                          k                              ,                              i                                                                                                      =                                                  -                          1                                                                                      ⁢                                          exp                      ⁢                                              {                                                  -                                                                                    m                              conv                                                        ⁡                                                          (                                                                                                s                                  |                                  r                                                                ,                                                                  R                                  v                                                                ,                                H                                                            )                                                                                                      }                                                                                            +                                  α                                      k                    ,                    i                                                                                                                          ≈                            ⁢                                                                    min                                                                  s                        ⁢                                                  :                                                ⁢                                                  b                                                      k                            ,                            i                                                                                              =                                              -                        1                                                                              ⁢                                                            m                      conv                                        ⁡                                          (                                                                        s                          |                          r                                                ,                                                  R                          v                                                ,                        H                                            )                                                                      -                                                                                                      ⁢                                                                    min                                                                  s                        ⁢                                                  :                                                ⁢                                                  b                                                      k                            ,                            i                                                                                              =                      1                                                        ⁢                                                            m                      conv                                        ⁡                                          (                                                                        s                          |                          r                                                ,                                                  R                          v                                                ,                        H                                            )                                                                      +                                  α                                      k                    ,                    i                                                                                                                          =                            ⁢                                                                    min                                                                  s                        ⁢                                                  :                                                ⁢                                                  b                                                      k                            ,                            i                                                                                              =                                              -                        1                                                                              ⁢                                      {                                                                                            (                                                      r                            -                            Hs                                                    )                                                H                                            ⁢                                                                        R                          v                                                      -                            1                                                                          ⁡                                                  (                                                      r                            -                            Hs                                                    )                                                                                      }                                                  -                                                                                                      ⁢                                                                    min                                                                  s                        ⁢                                                  :                                                ⁢                                                  b                                                      k                            ,                            i                                                                                              =                      1                                                        ⁢                                      {                                                                                            (                                                      r                            -                            Hs                                                    )                                                H                                            ⁢                                                                        R                          v                                                      -                            1                                                                          ⁡                                                  (                                                      r                            -                            Hs                                                    )                                                                                      }                                                  +                                  α                                      k                    ,                    i                                                                                                          (        2        )            where bk,i denotes the i-th bit of the k-th symbol sk and αk,i≡ln [p(bk,i=1)/p(bk,i=−1)] denotes a priori information about the bit bk,i, if available.
When the impairment signal v is dominated by a single interferer, the statistics of v can be far from Gaussian. In this case, the conventional demodulation metric (mconv) defined by Equation (1) is not the best metric for demodulating transmitted symbols s, and Equation (2) is not the best formula for computing soft bit information. In D. Hui and R. Ramesh, “Maximum likelihood sequence estimation in the presence of constant envelope interference,” Proc. IEEE VTC-Fall, 2003, a modified demodulation metric was introduced for the case when the dominant interferer employs a constant-envelope modulation such as the Gaussian Minimum Shift Keying (GMSK) modulation used in Global System for Mobile Communications (GSM) cellular networks. However, in the latest generations of cellular standards, such as High Speed Packet Access (HSPA) and Long Term Evolution (LTE), higher-order, non-constant-envelope modulation, such as Quadrature Amplitude Modulation (QAM), is used in order to achieve higher user throughput. As such, there is a need for an enhanced demodulation metric for demodulation in the presence of a non-constant envelope modulated interfering signal.